One of the most notorious crimes in recent American history returned to transfix the country yesterday with the arrest of a 41-year-old man in connection with the 1996 murder of the six-year-old child beauty queen, JonBenet Ramsey.

In tumultuous scenes in Bangkok, John Mark Karr told a press conference that he was with the the girl when she died.

"I loved JonBenet," he said in a quiet but firm voice. "I was with JonBenet when she died. She died accidentally."

Asked if he was innocent, he replied: "No." He continued: "It's very important for me that everyone knows that I love her very much, and that her death was unintentional, and that it was an accident."

The Boxing Day 1996 murder of JonBenet Ramsey gripped America. Pictures of the blond six-year-old dressed in beauty pageant outfits and videos of her dancing played ceaselessly on television news programmes after her bound and bludgeoned body was found by her father in the basement of the family home in Boulder, Colorado.

Mr Karr was arrested by Thai immigration officials and US agents on a US federal warrant at the Blooms hotel, a budget hotel frequented by long-stay travellers in Bangkok. After packing his belongings for an hour, he was taken away dressed in a turquoise polo shirt and khaki trousers. As he was led away he told reporters that the girl's death was "not at all what it seems to be".

American immigration authorities said Mr Karr would be returned to the US within the week. Thai authorities revoked his visa.

The arrest came after months of investigation. It is thought that Mr Karr, who family members said was researching a book on men who kill girls, contacted Professor Michael Tracey of the University of Colorado about the case four years ago. Prof Tracey has produced three British television documentaries about the case. He alerted Boulder police, who monitored email exchanges between him and Mr Karr. It is thought the arrest this week was triggered when he began working in Thailand as a teacher on Tuesday.

JonBenet's mother died in June of ovarian cancer. Her husband, John Ramsey, told a Colorado newspaper that she had been aware of the investigation before her death. "Patsy was aware that authorities were close to making an arrest in the case and had she lived to see this day would no doubt have been as pleased as I am with today's development, almost 10 years after our daughter's murder," he said.

But he cautioned against any leap to judgment and urged the media and authorities to respect Mr Karr's right to a presumption of innocence.

"Do not jump to conclusions," he said. "Do not speculate. Let the justice system take its course."

David Mills, who co-produced the British television documentaries with Prof Tracey, said he had spoken to Mr Ramsey earlier in the week. "He was very dignified in a quiet way," Mr Mills said. "He would be horrified if the same thing happened to Karr that happened to him and Patsy."

As he was arrested, Mr Karr said he had written to JonBenet's mother earlier this year shortly before she died. "I conveyed to her many things, among them that I am so very sorry for what happened to JonBenet," he said.

He will be taken within the week to Colorado, where he will face charges of first-degree murder, kidnapping and child sexual assault.

The public and media's fascination with the story was fuelled by the suggestion that her parents were involved in JonBenet's murder. The images of a little girl dressed up for beauty pageants, wearing full make-up and gaudy outfits reminiscent of a Las Vegas showgirl, fuelled a bout of introspection about the sexualisation of childhood in modern America.

Patsy Ramsey had been a beauty queen in her youth, as had her mother; her desire that her daughter follow her example raised suspicions in many minds about the family's motives. During her short career JonBenet was crowned Little Miss Colorado, National Tiny Miss Beauty, Colorado State All-Star Kids Cover Girl and America's Royale Little Miss.

Police initially focused on the parents, suggesting to the media that their story did not make sense. A 370-word ransom note found at the scene contained many bizarre details that befuddled investigators. It had been hand-written with one of JonBenet's felt-tip pens on notepaper from the family home. It suggested that the person who wrote it had an intimate knowledge of the family: it demanded a ransom of $118,000, the amount Mr Ramsey had recently received as a work bonus. It also said the kidnappers belonged to a "small foreign faction".

But police in Boulder were criticised for their conduct of the investigation, and disputes broke out between police and the local district attorney's office.

JonBenet's body was found by her father after police allowed him to enter the house unaccompanied. He brought the body up from the basement and laid it under the Christmas tree, while his wife screamed for help from Jesus to raise her daughter as Lazarus had been raised from the dead.

In a recent interview with the Observer, Prof Tracey speculated that the case gripped America because it had everything going for it: "Sex, sleaze, the rich father, the American dream gone bad ... It was a combination of voyeurism, resentment, anger, irrationality, a cultural viciousness. It was Greek - a lot of people focused on it as a kind of catharsis."

Asked why the Ramsey family had become a target of police and an obsession with the media, he responded: "It's in the hate. Hate the Ramseys, you feel better. This was pretty close to a conspiracy."

As time passed the parents were able to clear their name, although there is still a common presumption that they were involved in their daughter's death.

In October 1999 a grand jury refused to hand down an indictment against Patsy Ramsey. In 2002 the Ramseys reached an out-of-court settlement in a libel suit they had brought against a detective involved in the case who had written a book alleging that they had been involved in the murder. In 2003 an extensive court ruling agreed with the parents that the murder was the work of an intruder.

But each episode enabled the media to obsess about the strange case of the child beauty queen and her religious parents. Writing in Wednesday's Rocky Mountain News, one of two major newspapers in Denver, columnist Mike Littwin described coverage of the case as voyeurism.

"This was a story that stayed alive only as long as there was one more opportunity to show the runway tape of the child in a sadly grotesque beauty contest. This was a story that stayed alive because of the rich parents who were obviously guilty of something - if only of bad taste, or of hiring the wrong press agent," he wrote.

For Mr Mills, who hopes to make a further documentary about the murder, the case exposes deep faults with the American media and the US justice system. "The presumption of innocence was tragically denied to John and Patsy Ramsey," he said. "We now know that the police had nothing. Everybody assumed this couple was guilty. It was Mickey Mouse journalism. The media turned the story into a commercial product, nobody really cared whether it was true or not."